Premixer with diluent fluid and fuel tubes having chevron outlets

ABSTRACT

A premixer includes an air tube formed in a burner tube defining a longitudinal axis, and a coaxially disposed fuel tube with a turbulence enhancing chevron outlet. The fuel tube may include an exterior tube and an interior tube with the interior tube, the exterior tube or both having chevron outlets. The chevron outlets may be tapered and notched.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to co-pending U.S. patent applicationsentitled “PREMIXER WITH FUEL TUBES HAVING CHEVRON OUTLETS”, Ser. No.______, filed concurrently herewith.

GOVERNMENT RIGHTS

This invention was made with Government support under contract numberDE-FC26-05NT42643 awarded by the Department of Energy. The Governmenthas certain rights in this invention.

TECHNICAL FIELD

The subject matter disclosed herein relates to premixing devices forinjection nozzles and more particularly to a premixer having coaxialfuel tube for enhanced mixing, flame holding and flashback resistance.

BACKGROUND

Gas turbine manufacturers are continuously improving the emissionperformance of gas turbines. The primary pollutant produced by gasturbines are oxides of nitrogen (NOx), carbon monoxide (CO) and unburnedhydrocarbons. NOx emissions depend upon the maximum temperature in thecombustor and the residence time for the reactants. One known method ofcontrolling the temperature in the combustor is to premix fuel and airto a lean mixture prior to combustion. Such premixing tends to reducecombustion temperatures and undesirable NOx emissions. In these systemsfuel is mixed with air using a premixing device that is upstream of acombustion zone for creating a premixed flame at lean conditions toreduce emissions from the combustor. Ideally, the flame should becontained inside of the combustor downstream of the fuel/air premixingpassages. However, premixing devices are susceptible to flashback.During flashback, the fuel and air mixture in the premixing passagescombusts. The flashback condition generally occurs when a flame travelsupstream from the main burning zone into the premixing zone. Seriousdamage may occur to the combustion system when flame holding orflashback occurs. Similarly, the flame may develop on or near surfaces,which can also result in damage due to the heat of combustion. Thisphenomenon is generally referred to as flame holding. For example, theflame holding may occur on or near a fuel nozzle in a low velocityregion. In particular, an injection of a fuel flow into an air flow maycause a low velocity region near the injection point of the fuel flow,which can lead to flame holding.

Typically, it is difficult to control flame holding in premixingdevices. In some combustors, the average velocity of fuel/air mixturemay be increased within a mixing region of the premixing device forenhancing the flame holding margins in such devices. However, thisresults in a relatively high pressure drop across the combustor, therebydecreasing the combustor efficiency.

BRIEF DESCRIPTION OF THE INVENTION

In accordance with one exemplary non-limiting embodiment, the inventionrelates to a premixer having a fuel plenum, a burner tube, and an airtube formed in the burner tube. A fuel tube having a chevron outlet iscoaxially disposed inside the air tube. An air source is coupled to theair tube. The premixer includes a bell mouth disposed at least partiallyabout the burner tube and coupled to the air source, the bell mouthadapted to direct air into the air tube.

In another embodiment, the invention relates to a combustor with a fuelsource, an air source, and a premixer. The premixer includes a burnertube and an air tube formed in the burner tube and coupled to the airsource, and a fuel tube with a turbulence enhancing chevron outletdisposed inside the air tube and coupled to the fuel source. Thepremixer also includes a bell mouth disposed at least partially aboutthe burner tube and coupled to the air source the bell mouth adapted todirect air into the at least one air tube.

In another embodiment, an apparatus is provided that includes a fuelsource, a first fuel tube coupled to the fuel source and a chevronoutlet for enhancing turbulence of fuel flowing through the first fueltube. The apparatus also includes an air tube surrounding the first fueltube an air source, and an air directing component coupled to the airsource and adapted to direct air into the air tube.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the present invention will be apparentfrom the following more detailed description of the preferredembodiment, taken in conjunction with the accompanying drawings whichillustrate, by way of example, the principles of certain aspects of theinvention.

FIG. 1 is a perspective view of an embodiment of a premixer.

FIG. 2 is a cross-sectional view of an embodiment of a premixer.

FIG. 3 is a cross-sectional view of an embodiment of a premixer.

FIG. 4 is a perspective view of an embodiment of a fuel tube.

FIG. 5 is a cross-sectional view of an embodiment of a fuel tube.

FIG. 6 is a cross-sectional view of an embodiment of a fuel tube.

FIG. 7 is a cross-sectional view of an embodiment of a fuel tube.

FIG. 8 is a perspective view of an embodiment of a fuel tube.

FIG. 9 is a cross-sectional view of an embodiment of a fuel tube.

FIG. 10 is a perspective view of an embodiment of a fuel tube.

FIG. 11 is a cross-sectional view of an embodiment of a fuel tube.

FIG. 12 a perspective view of an alternate embodiment of a premixer.

FIG. 13 is a cross-sectional view of an embodiment of a premixer.

FIG. 14 is a cross-sectional view of an embodiment of a fuel tube.

FIG. 15 is a cross-sectional view of an embodiment of a fuel tube.

FIG. 16 is a cross-sectional view of an embodiment of a fuel tube.

FIG. 17 is a cross-sectional view of an embodiment of a fuel tube.

FIG. 18 is a cross-sectional view of an embodiment of a fuel tube.

FIG. 19 is a cross-sectional view of an embodiment of a fuel tube.

FIG. 20 is a cross-sectional view of an embodiment of a nozzle assembly.

FIG. 21 is a cross-sectional view of an embodiment of a nozzle assembly.

FIG. 22 is a schematic of an embodiment of a turbine system with anozzle assembly.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a premixer 10 as may be described herein. The premixer 10may be used in a combustor (not shown) and in similar devices. Thepremixer 10 may include a fuel conduit 11 coupled to a fuel plenum 12.The fuel plenum 12 is in fluid communication with one or more fuel tubes13. The fuel plenum 12 may have any desired size or configuration. Anynumber of fuel tubes 13 may be used herein. The fuel tubes 13 may haveany desired size or configuration.

The premixer 10 also may include a burner tube 14. The burner tube 14may include one or more air tubes 15 extending through the burner tube14. Any number of the air tubes 15 may be used. The air tubes 15 mayhave any desired size or configuration. The air tubes 15 may extend froma bell mouth 16 facing the fuel plenum 12 to an opposite burner tube 14.The air tubes 15 may have a larger diameter as compared to the fueltubes 13.

The fuel plenum 12 and the burner tube 14 may be separated by a numberof spacers 18. Any number of spacers 18 may be used. The spacers 18 mayhave any desired size or configuration. The spacers 18 may be attachedto a burner tube flange 20 on one end and a fuel plenum flange 22 on theother. The fuel plenum flange 22 and the burner tube flange 20 may haveany desired size or configuration. Other types of connection means maybe used herein. The spacing between the fuel plenum 12 and the burnertube 14 may vary.

A number of the fuel tubes 13 may extend from the fuel plenum 12 into anumber of the air tubes 15 within the burner tube 14. The fuel tubes 13have a downstream end portion 17 (shown in FIG. 2) that may be providedwith a variety of geometries. Fuel thus may enter the fuel plenum 12 viafuel conduit 11 and may be distributed to the fuel tubes 13. The fuelmay then be injected from the downstream end portion 17 (shown in FIG.2) of the fuel tube 13 into an air stream in each of the air tubes 15 soas to mix the fuel and air. Likewise, the air flow may be guided betweenthe fuel plenum 12 and the bell mouth 16 and into the air tubes 15. Someor all of the air tubes 15 may have one of the fuel tubes 13 positionedtherein. The premixer 10 thus may use a plurality of air tubes 15 with aplurality of fuel tubes 13 so as to create multiple co-flow jets of airand fuel. As used herein, the term “co-flow” means the fuel and air flowin the same direction at the point of injection of the fuel. The sizeand number of the fuel tubes 13, and air tubes 15 may vary. Otherconfigurations of the premixer 10 may be used herein.

Illustrated in FIG. 2 is an embodiment of a premixer 10 having a fuelconduit 11, a fuel plenum 12, and a plurality of fuel tubes 13 coupledto the fuel plenum 12. A plurality of air tubes 15 are formed in theburner tube 14. A bell mouth 16 is disposed around the burner tube 14and functions to force air into the plurality of air tubes 15. One ofthe plurality of fuel tubes 13 may be disposed coaxially within one ofthe plurality of air tubes 15 which feed fuel and air into a mixingportion 25 of air tubes 15 wherein the air and fuel are mixed. As usedherein, the terms “coaxial” and “coaxially” means that the fuel tubes 13and the air tubes 15 substantially share a common longitudinal axis orhave longitudinal axes that are substantially parallel to each other.The terms “coaxial” and “coaxially” are intended to be interpretedbroadly and include small variations in the angles formed by therespective longitudinal axes of the air tubes 15 and the fuel tubes 13and minor offsets between the longitudinal axes. The arrangement of thefuel tubes 13 and the air tubes 15 enable the creation of a flow jet ofair and fuel within the mixing portion 25. Multiple flow jets of air andfuel may be obtained using multiple pairs of fuel tubes 13 and air tubes15.

Illustrated in FIG. 3 is an embodiment of a premixer 10 wherein the airtubes 15 are provided with a tapered upstream section 27. The term“tapered”, as used herein means a structure that is not uniform indimension. For example, the tapered upstream section 27 of one of theair tubes 15 may have a larger diameter than the diameter of thedownstream section of the air tubes 15. This configuration helps toaccelerate the airflow for better fuel and air mixing in the mixingportion 25 of the air tubes 15 and to obtain an enhanced margin offlashback/flame holding and faster fuel/air mixing that can shorten thelength of the body of the air tubes 15 and result in reduced NOx.

In operation, fuel enters the fuel plenum 12, and is conveyed into thefuel tubes 13. Air is entrained by bell mouth 16 and is conveyed to theair tubes 15. The fuel and air mix in the mixing portion 25 of the airtubes 15. The fuel exiting the fuel tubes 13 is provided with enhancedturbulence resulting from the various chevron configurations of thedownstream end portion 17 of the fuel tubes 13 (described in detailbelow) thereby shortening the length required to achieve adequate mixingin the mixing portion 25 of the air tubes 15.

FIGS. 4 and 5 illustrate the downstream end portion 17 of one of thefuel tubes 13. As used herein, the term “downstream” means in thedirection of flow of the fuel/air mixture. The fuel tubes 13 may have acylindrical end 31 that is provided with a V-shaped notch 33. TheV-shaped notch 33 induces and enhances the turbulence of the fuel/airflow beyond the downstream end portion 17 of the fuel tubes 13 to enablethe mixing of fuel and air in the mixing portion 25 of the air tubes 15within a shorter interval and shorter length along the air tubes 15. TheV-shaped notch 33 defines a turbulence enhancing chevron outlet 34 thatincreases the turbulence intensity of the fuel/air mixture and widensthe mixing layer beyond the point of the fuel injection. Theconfiguration of the downstream end portion 17 of the fuel tubes 13 mayvary as shown from the following examples.

FIGS. 6 and 7 illustrate alternate embodiments of the downstream endportion 17 of one of the fuel tubes 13. In both embodiments, a straightnotch 35 with a straight inboard portion 36 is provided. Thecircumferential dimensions of the straight notch 35 may vary as shown.

It should be noted that the illustrated embodiments of the turbulenceenhancing chevron outlet 34 represent general examples and are notintended to be limiting. For example, other shapes of a turbulenceenhancing chevron outlet 34 may be used. Similarly the dimensions of theturbulence enhancing chevron outlet 34 may be varied without departingfrom the spirit and scope of the invention as described and claimedherein. Other geometries and means for enhancing the turbulence of thefuel exiting the fuel tube 13 may be used as may be apparent to one ofordinary skill in the art in light of the teachings herein.

FIGS. 8 and 9 illustrate the downstream end portion 17 of one of thefuel tubes 13 that is provided with a downstream end 43 that is tapered(tapered end 37). The tapered end 37 may be provided with a V-shapednotch 33.

FIGS. 10 and 11 illustrate the downstream end of one of the fuel tubes13 that is provided with a tapered end 37 and with a U-shaped notch 38.

As one of ordinary skill in the art will appreciate, different numbersand shapes of notches (chevrons) and different angles and lengths oftaper for the downstream end portion 17 of fuel tubes 13 may beselectively applied to form other possible embodiments of the presentinvention.

FIGS. 12 and 13 show an alternative embodiment of a premixer 39 as isdescribed herein. In this embodiment, the premixer 39 includes adownstream plenum 40 and an upstream plenum 41. The downstream plenum 40and the upstream plenum 41 may have any desired size or configuration.In one embodiment, the upstream plenum 41 is used as a plenum for fueland the downstream plenum 40 is used as a plenum for diluent. Thepremixer 39 includes one or more coaxial tube assemblies 42. Each of theone or more coaxial tube assemblies 42 includes an interior tube 44(shown in FIG. 13) that may extend from the upstream plenum 41 and passthrough the downstream plenum 40. Likewise, an exterior tube 45 mayextend from the downstream plenum 40. The exterior tube 45 may surroundthe interior tube 44. The size and number of coaxial tube assemblies 42may vary. Other configurations may be used herein.

The premixer 39 also includes a burner tube 14 coupled to a combustionchamber (not shown). As above, the burner tube 14 includes a number ofair tubes 15 extending through the burner tube 14. The size and numberof the air tubes 15 may vary. The air tubes 15 may extend from the bellmouth 16 to a downstream end 46 of burner tube 14. Some of the air tubes15 may have interior tube 44 extending therein while others may haveexterior tube 45 extending therein. The air tubes 15 may have a largerdiameter as compared to the coaxial tube assemblies 42.

The premixer 39 also may include a number of spacers 18 that separatethe downstream plenum 40 and the burner tube 14. Any number of spacers18 may be used. The spacers 18 may have any desired size orconfiguration. The downstream plenum 40 may have a fuel plenum flange 22while the burner tube 14 may have a burner tube flange 20. Fuel plenumflange 22 and burner tube flange 20 may have any desired size orconfiguration. Other types of connection means may be used herein. Thespacing between the downstream plenum 40 and the burner tube 14 mayvary.

Fuel or other types of flows thus may flow from the upstream plenum 41and the downstream plenum 40 through the coaxial tube assemblies 42, andmix with the air in the air tubes 15. The upstream plenum 41 anddownstream plenum 40 may be used with different arrangements of fuel,air, and diluents. For example, the downstream plenum 40 may be usedwith a diluent such as nitrogen (N₂) while the upstream plenum 41 may beused with a fuel such as hydrogen (H₂), or methane CH₄), or acombination of both. Alternatively, the diluent flow may be or include aless reactive fuel. This arrangement of diluent and fuel may create adiluent inert sheath surrounding the flow of fuel. Such a diluent inertsheath passing through the air tubes 15 may prevent flame holding insideair tubes 15.

Other fuel and air mixing mechanisms may be used herein. For example,the upstream plenum 41 may be arranged with air while the downstreamplenum 40 may have a mixture of hydrogen and nitrogen. Likewise, theupstream plenum 41 may use nitrogen while the downstream plenum 40 mayuse combinations of hydrogen and nitrogen. Other arrangements anddifferent types and combinations of air, fuel, and diluent may be usedherein.

The premixer 10, and premixer 39 described herein (in FIGS. 2 and 13respectively) thus may use multiple fuel and air tubes 15 to createmultiple co-flow jets of air and fuel. Fuel and air mixing may beenhanced due to the length versus the diameter of the air tubes 15. Thepremixer 10 and the premixer 39 also may use jets of fuel in combinationwith a sheath of diluent and/or diluent and air. The multiple jets alsoprovide an increased flame holding margin due to the increased axialcomponent of the fuel flow to air flow and by compartmentalizing theconventional burner tube into the multiple tubes. Local quenching alsomay be induced by the inert nitrogen, other diluents, and/or a high flowof air so as to limit flame holding. Very low emissions thus may beachieved by the good mixing caused by injecting the fuel into each airtube 15.

Illustrated in FIG. 13 is a cross-section of premixer 39. As with theprevious embodiment discussed with regard to FIG. 1, the premixer 39includes an upstream plenum 41 and a downstream plenum 40 withassociated fuel conduits 11 and diluent fluid conduits 47, respectively.The premixer 39 may include one or more coaxial tube assemblies 42having an interior tube 44 and an exterior tube 45. In one embodiment,the interior tube 44 may be coupled to the upstream plenum 41. Bellmouth 16 forces air into a plurality of air tubes 15. The interior tube44 and exterior tube 45 are disposed coaxially within air tubes 15. Theair and fuel are mixed in mixing portion 25 of the air tubes 15. In oneembodiment, the exterior tube 45 may be coupled to downstream plenum 40which is in turn coupled to the diluent fluid conduit 47 that providesdiluent fluid such as air or an inert fluid such as N₂. In anotherembodiment, the interior tube 44 may be coupled to the downstream plenum40 as a plenum for diluent fluid and the exterior tube 45 may be coupledto the upstream plenum 41 as a plenum for fuel.

In the premixer 39 illustrated in FIG. 13, fuel enters the upstreamplenum 41 through the fuel conduit 11. A diluent fluid is provided tothe downstream plenum 40 through the diluent fluid conduit 47. Theinterior tube 44 of the coaxial tube assemblies 42 may be coupled to theupstream plenum 41, and the exterior tube 45 may be coupled to thedownstream plenum 40. The fuel and diluent fluid exiting the coaxialtube assemblies 42 may be provided with enhanced turbulence resultingfrom the various configurations of the downstream end 48 of the coaxialtube assemblies 42, thereby shortening the length of air tubes 15required to achieve adequate mixing. As one of ordinary skill in the artwill appreciate, the fuel may also be transported through exterior tube45, and the diluent fluid may be transported through the interior tube44.

The downstream end 48 of the coaxial tube assemblies 42 may have one ofa variety of configurations as illustrated in FIGS. 14-19. FIG. 14illustrates an embodiment of one of the coaxial tube assemblies 42 wherethe downstream end 48 of the exterior tube 45 has a tapered end 49, andthe interior tube 44 has a protruding straight end 51 with a V-shapednotch 33. FIG. 15 illustrates an embodiment where the downstream end 48of the exterior tube 45 has a tapered end 49, and the interior tube 44also has a tapered end 53. FIG. 16 illustrates an embodiment where thedownstream end 48 of the exterior tube 45 has a straight end 55, and theinterior tube 44 has a tapered end 53 that protrudes from the downstreamend 48 of the exterior tube 45. FIG. 17 illustrates an embodiment wherethe downstream end 48 of the exterior tube 45 has a tapered end 49, andthe interior tube 44 has a straight end 57 mounted flush with the end ofexterior tube 45. FIG. 18 illustrates an embodiment where the downstreamend 48 of the exterior tube 45 has a tapered end 49, and the interiortube 44 has a tapered end 59 mounted flush with the end of the exteriortube 45. FIG. 19 illustrates an embodiment where the downstream end 48of the exterior tube 45 has a straight end 55, and the interior tube 44has a tapered end 59 mounted flush with the end of the exterior tube 45.Although the tapered end 49 and tapered end 59 are shown as a straighttaper, other types of tapers may be used without departing from thespirit and scope of the invention as described and claimed herein.

The various embodiments of the downstream end portion 17 of the fueltubes 13 (Shown in FIG. 2) and the downstream end portion 48 of thecoaxial tube assemblies 42 (shown in FIG. 13) serve to minimize flameholding/flashback problems. The premixer 10 will typically use one ormore fuel tubes 13 within air tubes 15 to create multiple co-flow jetsof air and fuel to prevent flame holding at the fuel injection location.The coaxial tube assemblies 42 provide fuel in combination with a sheathof annular diluent fluid such as N₂ to premix the air and fuel withinthe air tubes 15. The mixing lengths can be improved (shortened) byproviding fuel tubes 13 or coaxial tube assemblies 42 with an enhancedturbulence capability such as, for example, through the use of chevronoutlets such as V-shaped notch 33 (illustrated in FIG. 5) or U-shapednotch 38 (illustrated in FIG. 11). The enhanced turbulence capabilityprovides fast mixing within short length in the air tubes 15.

Illustrated in FIG. 20 is a combustor 60 having a plurality of premixers10 that inject a fuel/air mixture into a combustion chamber 63 with acombustor liner 64. Each of the premixers 10 may be coupled to a fuelsource 65. Air (shown by dashed arrows) from air source 67 enters theair tubes 15 and mixes with fuel injected through the fuel tubes 13before entering the combustion chamber 63. The premixers 10 may besupported by a housing 71.

Illustrated in FIG. 21 is a combustor 75 having a liner 73. Thecombustor 75 includes a plurality of premixers 39. The premixers 39 maybe coupled to one or more fuel sources 65 and at least one source ofdiluent fluid 79. Fuel and diluent fluid are conveyed through coaxialtube assemblies 42. Air (shown by dashed arrows) from air source 67enters the air tubes 15 and mixes with fuel and diluent fluids injectedthrough the coaxial tube assemblies 42 before entering the combustionchamber 63.

FIG. 22 is a schematic illustrating the environment in which thepremixer 10 may be implemented. A gas turbine 81 may include acompressor 83 coupled to a compressed air conduit 85. The compressed airconduit 85 supplies compressed air to a combustor 89 coupled to anexhaust conduit 91. Exhaust from exhaust conduit 91 drives a turbine 93which may drive a shaft 97 providing power to a load 103. The combustor89 may include a nozzle assembly 99 with a premixer 10 (such asillustrated in FIG. 2) coupled to a fuel source 101. The premixer 10 maybe used to provide multiple co-flow jets of air and fuel. The assembliesmay be used for high H₂ combustion to improve the mixing of the high H₂fuel and air without the cost of system pressure drop, and to providereduced NOx and flashback probability.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention.Where the definition of terms departs from the commonly used meaning ofthe term, applicants intend to utilize the definitions provided herein,unless specifically indicated. The singular forms “a”, “an” and “the”are intended to include the plural forms as well, unless the contextclearly indicates otherwise. It will be understood that, although theterms first, second, etc. may be used to describe various elements,these elements should not be limited by these terms. These terms areonly used to distinguish one element from another. The term “and/or”includes any, and all, combinations of one or more of the associatedlisted items. The phrases “coupled to” and “coupled with” contemplatesdirect or indirect coupling.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to practice the invention, including making and using any devices orsystems and performing any incorporated methods. The patentable scope ofthe invention is defined by the claims, and may include other examplesthat occur to those skilled in the art. Such other examples are intendedto be within the scope of the claims if they have structural elementsthat do not differ from the literal language of the claims, or if theyinclude equivalent structural elements.

What is claimed:
 1. A premixer, comprising: a fuel plenum; a burnertube; an air tube formed in the burner tube; a first tube disposedcoaxially inside the air tube; a second tube disposed coaxially insidethe first tube at least one of the first tube and second tube having achevron outlet, at least one of the first tube and second tube coupledto the fuel plenum, and disposed coaxially inside the air tube; an airsource coupled to the air tube; and a bell mouth disposed at leastpartially about the burner tube and coupled to the air source, the bellmouth adapted to direct air into the air tube.
 2. The premixer of claim1, wherein the chevron outlet comprises a notch at a downstream end ofthe second tube.
 3. The premixer of claim 2 wherein the notch comprisesa notch selected from among a group consisting of a V shaped notch, a Ushaped notch and a straight notch.
 4. The premixer of claim 1, whereinat least one of the first tube and second tube having a tapereddownstream end.
 5. The premixer of claim 1, wherein the first tube has achevron outlet and the second tube does not have a chevron outlet. 6.The premixer of claim 1, wherein the second tube has a chevron outletand the first tube does not have a chevron outlet.
 7. The premixer ofclaim 5, further comprising a source of diluent fluid coupled to one ofthe first tube and second tube.
 8. A combustor, comprising: a fuelsource; an air source; and a plurality of premixers comprising: a burnertube; an air tube formed in the burner tube and coupled to the airsource; a first tube disposed coaxially inside the air tube; a secondtube disposed coaxially inside the first tube; at least one of the firsttube and second tube having a turbulence enhancing chevron outlet; andat least one of the first tube and second tube coupled to the fuelsource.
 9. The combustor of claim 8 each premixer further comprises abell mouth disposed at least partially about the burner tube and coupledto the air source the bell mouth adapted to direct air into the airtube.
 10. The combustor of claim 8, wherein the turbulence enhancingchevron outlet comprises a notch at a downstream end of the first tube.11. The combustor of claim 8, wherein the turbulence enhancing chevronoutlet comprises a notch at a downstream end of the second tube.
 12. Thecombustor of claim 8 wherein the first tube comprises a tube with atapered downstream end and a notch on the tapered downstream end. 13.The combustor of claim 8 wherein the second tube comprises a tube with atapered downstream end and a notch on the tapered downstream end. 14.The combustor of claim 12, wherein at least one of the first tube or thesecond tube is tapered.
 15. The combustor of claim 12, furthercomprising a source of diluent fluid coupled to the first tube andwherein the fuel source is coupled to the second tube.
 16. The combustorof claim 12, further comprising a source of diluent fluid coupled to thesecond tube and wherein the fuel source is coupled to the first tube.17. An apparatus, comprising: a fuel source; a first tube; a second tubedisposed within the first tube; wherein one of the first tube or secondtube is coupled to the fuel source and at least one of the first tube orsecond tube has a chevron outlet for enhancing turbulence; an air tubesurrounding the first tube; an air source; and an air directingcomponent coupled to the air source and adapted to direct air into theair tube.
 18. The apparatus of claim 17, wherein the chevron outletcomprises one selected from among a group consisting of a V-shaped notchon the first tube, a U shaped notch on the first tube, and a straightnotch on the first tube.
 19. The apparatus of claim 17, wherein at leastone of the first tube or second tube is tapered.
 20. The apparatus ofclaim 17, further comprising: a diluent source; and wherein the firsttube is coupled to the fuel source and the second tube is coupled to thediluent source.
 21. The apparatus of claim 17, further comprising: adiluent source; and wherein the second tube is coupled to the fuelsource and the first tube is coupled to the diluent source.